The overall objective of this proposal is to study the activation, proliferation and differentiation of primordial germ cells, by investigating the factors that regulate the outgrowth of experimental germ cell derived teratomas in fetal genital ridges. To this end we shall combine in vitro cultivation of gonads and isolated germ cells with transplantation of these organs and cells into adult histocompatatible mice. Immunohistochemical, enzyme and ultrastructural techniques will be used to monitor the activation and differentiation of germ cells in vivo and in vitro. In vivo transplantation studies will be used to determine the tumorigenicity of germ cells derived cells. Factors that inhibit or promote the growth and differentiation of germ cells and their derivatives in vivo and in vitro will be defined.